_content/doc/install/source: mention GORISCV64

Go 1.23 added support for the GORISCV64 environment variable that
can be used to instruct the compiler to target a specific RISC-V
application profile.  By default, GORISCV64 is set to rva20u64
allowing the compiler to target the mandatory extensions in the
RVA20U64 profile, essentially rv64gc.  By setting GORISCV64 to
a later profile, such as rva22u64, the compiler can use additional
instructions, such as those provided by Zba, Zbb and Zbs extensions,
to reduce code size and improve performance.

This is all already documented in "go help environment" but not in
the "Installing Go from source" web page.  This commit fixes that
oversight.

Updates golang/go#61476

Change-Id: Ie4aa1804c64b5abf5a3089e1e201d85af65e6119
Reviewed-on: https://go-review.googlesource.com/c/website/+/633615
Auto-Submit: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: markdryan

_content/doc/install/source.html

@@ -763,6 +763,15 @@ <h2 id="environment">Optional environment variables</h2>
 </ul>
 </li>
 
+<li><code>$GORISCV64</code> (for <code>riscv64</code> only)
+<p>
+This variable sets the RISC-V user-mode application profile for which to compile. The default is <code>rva20u64</code>.
+</p>
+<ul>
+	<li><code>GORISCV64=rva20u64</code>: only use RISC-V extensions that are mandatory in the <a href="https://github.com/riscv/riscv-profiles/blob/main/src/profiles.adoc#51-rva20u64-profile">RVA20U64</a> profile</li>
+	<li><code>GORISCV64=rva22u64</code>: only use RISC-V extensions that are mandatory in the <a href="https://github.com/riscv/riscv-profiles/blob/main/src/profiles.adoc#rva22u64-profile">RVA22U64</a> profile</li>
+</ul>
+</li>
 
 <li><code>$GOWASM</code> (for <code>wasm</code> only)
 	<p>